The purpose of this study is to identify genes (1) which are aberrantly expressed in diseased human skin tissue, particularly from patients with psoriasis and (2) genes expressed during specific delayed type hypersensitivity reactions. Since, blood cells accumulate in skin, it is also necessary to study genes expressed in blood-derived cells (particularly white blood cells). Skin tissue and blood will be collected for laboratory analysis. As normal values for gene expression must be established in skin, some individuals who participate in these studies will be normal volunteers and others may have inflammation unrelated to psoriasis. To atrifically induce and immune response, specific antigens and antigenic stimulants can be placed upon the skin. By comparing those genes expressed during defined cell mediated responses with genes in psoriatic plaques, we hope to further understand the underlying mechanism in this immune mediated disease.